Helical scan-type magnetic recording and playback systems for recording and playback of data signals on magnetic tape are well known. In these conventional systems, data signals are recorded on magnetic tape traveling at a specific speed. The tape is wrapped at an angle around an approximately 180-degree span of the surface of the outer circumference of a rotating cylindrical body. The recording and playback is performed by two rotary heads positioned 180 degrees apart on opposite sides of the rotating body. Normally, to increase the recording density in these conventional magnetic recording and playback systems, the two heads are set at different azimuth angles to permit the recording of more closely spaced data signal tracks on the magnetic tape, with either no guard bands, or very narrow bands, between tracks.
FIGS. 15(A) and 15(B) show enlarged views of portions of these recorded tracks and of the paths scanned by the rotary heads during normal playback, and during other-than-normal playback. As shown in FIGS. 15(A) and 15(B), on a magnetic tape 130, two tracks, T1 and T2, are recorded adjacent to each other, at an angle with respect to the longitudinal direction of the tape, with no guard band between them. Track T1 is formed as it is recorded by a rotary head Hm, with a portion of the magnetization reversals of the recorded signal positioned as indicated at 131 and 132. Track T2 is formed as it is recorded by a rotary head Hp, with a portion of the magnetization reversals of the recorded signal positioned as indicated at 133 and 134. The positions of the magnetization reversals shown at 131 and 132 indicate the recording wavelength. Similarly, the positions of the magnetization reversals shown at 133 and 134 indicate the recording wavelength.
Rotary heads Hm and Hp are magnetic heads that have gaps g1 and g2, respectively, positioned at an angle .alpha. with respect to the width direction of the track. In other words, rotating heads Hm and Hp are the same in that they both have the same azimuth angle of .alpha. degrees, but differ in that gap g1 is rotated away from the width direction of the track in a counterclockwise direction by an angle of .alpha. degrees, while gap g2 is rotated away from the width direction of the track in a clockwise direction by an angle of .alpha. degrees, the angle .alpha. being an acute angle of less than 90 degrees. For the purposes of this specification, the rotary head Hm having its gap g1 rotated in a counterclockwise direction away from the width direction of the track, shall be defined as a rotary head having a "minus" azimuth angle .alpha. (-.alpha. degrees). Similarly, the other rotary head Hp having a gap g2 rotated in a clockwise direction away from the width direction of the track shall be defined as a rotary head having a "plus" azimuth angle .alpha. (+.alpha. degrees).
During normal playback, rotary heads Hm and Hp reproduce signals from tracks T1 and T2 recorded either by these heads themselves, or by heads having the same azimuth angle, on a magnetic tape 130 traveling at the same speed as that at which it is being played back. Therefore, the scan paths of rotary heads Hm and Hp, as indicated by the dotted lines A1 and A2, respectively in FIG. 15(A), will be the same as the recorded paths of tracks T1 and T2. Accordingly, if the magnetization reversal intervals (record wavelengths) in tracks T1 and T2 are the same, the magnetization reversal interval (record wavelength) scan times for the two tracks will also be the same. Also, if the recorded data signal is a digital data signal, the data rates of the playback data from both tracks will be the same, regardless of the azimuth angle.
During special high-speed playback modes, however, rotary heads Hm and Hp reproduce signals from tracks T1 and T2 recorded either by these heads themselves, or by heads having the same azimuth angle, on a magnetic tape 130 traveling at a different speed from that at which it is being played back. Therefore, the scan paths of rotary heads Hm and Hp, as indicated by the dotted lines B1 and B2, respectively in FIG. 15(B), will be different from the recorded paths of tracks T1 and T2.
As can be seen in FIG. 15(B), the tracks are the same width, and the magnetization reversal interval (record wavelength) between track T1 magnetization reversal positions 131 and 132 is the same as that between track T2 magnetization reversal positions 134 and 135. However, the magnetization intervals at 131 through 134 have a slant angle, relative to the width of the tape, corresponding to that of the record rotary head gap. Therefore, when rotary heads Hm and Hp scan along paths B1 and B2, it will take rotary head Hm more time to scan through the magnetization reversal interval of track T1 than it takes rotary head Hp to scan through the magnetization reversal interval of track T2. Accordingly, when the recorded data signal is a digital signal, the data rate of the signal played back through head Hm will be lower than that played back through head Hp.
In VHS.RTM.-type VCRs, the rotary heads have an azimuth angle of only .+-.6 degrees. In 8-mm video, where 8-mm-wide magnetic tape is used to record and playback the data, the rotary head azimuth angle is .+-.10 degrees. Even in rotary head-type digital audiotape recorders (R-DAT) for recording and playing back digital audio signals, the rotary head azimuth angle is only .+-.20 degrees. Also, the tape speed in special playback modes in these systems is not that much greater than that in normal modes. For all practical purposes, then, for the above reasons, in these prior systems, it was possible to ignore the difference in playback times for heads Hm and Hp for the same magnetization reversal (record wavelength), due to the different azimuth angle during special high-speed playback modes, as described above. In other words, in the past, no corrective action was required.
In recent years, however, there has been a large amount of research in this area. New digital recorders, for example, have been developed for recording and playing back bit streams of digital data compressed in accordance with the MPEG format. Although these new recorders still use the same basic helical scan magnetic recording and playback system as that described above, they now use a larger rotary head azimuth angle of 30 degrees. Furthermore, they also use a playback tape speed that may be one hundred times greater than the speed at which the data was recorded. Therefore, this problem has now become large enough that it is no longer possible to ignore the difference in the data rates of the playback signals obtained from the two rotary heads due to the fact that they have different playback times for the same magnetization reversal which, as described above, is due to the change in the azimuth angle that occurs when the system goes to the special high-speed mode. There is now a problem then, in that it is no longer possible to efficiently reproduce playback data from the tape at high tape speeds in currently available systems.
The apparatus incorporating the principles of the present invention was conceived with the above problem in mind. It is an object of the present invention to provide a magnetic playback system capable of reproducing playback data with good efficiency.
It is a further object of the present invention to provide a magnetic playback system that can perform accurate playback of high-speed playback data in a low-cost configuration.